1. Field of the Invention
The present invention relates to a fuel vapor leakage inspection apparatus.
2. Description of the Related Art
Generally, a system is known for processing fuel vapor using an adsorbent configured to adsorb fuel vapor generated in a fuel tank. For example, granular activated carbon can be housed in an adsorption container, and the container will exhaust the fuel vapor adsorbed by the adsorbent to an intake pipe by means of a negative pressure in the intake pipe. The fuel vapor exhausted into the intake pipe is combusted in a combustion chamber. If leakage occurs in the fuel vapor processing system, the fuel vapor flows out into the atmosphere. Therefore, in such a case, it is necessary to inspect for the occurrence of leakage in the fuel vapor processing system. As a leakage inspection apparatus for the fuel vapor processing system, an apparatus for pressurizing or depressurizing a sealed fuel vapor path with a pump so as to detect the occurrence of leakage depending on a change in pressure after pressurization or depressurization has been known (for example, see Japanese Patent Laid-Open Publication No. Hei 11-351078).
In addition, other apparatuses for detecting the leakage based on a change in pump characteristics while the pump is being driven are known (for example, Japanese Patent Laid-Open Publications No. Hei 10-90107 and No. 2002-4959). However, if the leakage inspection is executed by pressurizing or depressurizing the sealed fuel vapor path by using pressure means such as a pump when the adsorbability of the adsorbent is lowered, for example, in the case where the adsorbent housed within the adsorption container is deteriorated, in the case where the adsorbent adsorbs a large amount of fuel vapor, and the like, the following problems occur.
In the case where the fuel vapor path is pressurized to execute the leakage inspection, when the fuel vapor path is depressurized after the pressurization of the fuel vapor path so as to exhaust the air in the fuel vapor path into the atmosphere, the fuel vapor present in the fuel vapor path is sometimes not adsorbed by the adsorbent but flows out into the atmosphere. On the other hand, in the case where the fuel vapor path is depressurized to execute the leakage inspection, when the air in the fuel vapor path is exhausted into the atmosphere so as to depressurize the fuel vapor path, all the fuel vapor present in the fuel vapor path sometimes cannot be adsorbed by the adsorbent and flows out into the atmosphere. Therefore, even if the leakage does not occur in the fuel vapor path itself, when the adsorbability of the adsorbent is lowered, there is a possibility that the fuel vapor flows out into the atmosphere when the leakage inspection is executed.
In the case where the leakage from the fuel vapor path is determined based on a path pressure in the fuel vapor path measured by pressurizing or depressurizing the fuel vapor path, if the fuel vapor adsorbed in a canister flows out to the atmosphere by an air flow generated by the pressurization or the depressurization, the pressure in the fuel vapor path changes in accordance with a concentration of the fuel vapor that flows out. Therefore, the fuel vapor leakage inspection apparatus suffers from the problem that the occurrence of leakage from the fuel vapor path cannot be precisely determined.